1. Field of the Invention
The present invention relates to a pressure switch, and more particularly to a pressure switch comprising an outer case which has a fluid inlet hole and a pressure equalizing hole being independent of the fluid inlet hole and being adapted to communicate with an outer space, a diaphragm which partitions an inner space of the outer case into a fluid pressure operating chamber communicating with the fluid inlet hole and a reference pressure operating chamber communicating with the pressure equalizing hole, electrical contact means, stored in the reference pressure operating chamber, for performing on/off-operation in accordance with deflection of the diaphragm caused by the pressure change of fluid flowed into the fluid pressure operating chamber through the fluid inlet hole, and an outer connection electrical terminal which is supported by the outer case for the electrical contact means.
2. Description of the Related Art
This type of pressure switch is used in, for example, an airconditioner for an automobile, and its structure is disclosed in, for example, Published Unexamined Japanese Patent No. 64-27137 and U.S. Pat. No. 4,939,321.
In the conventional pressure switch, plastic is selected as material of the diaphragm by taking a pressure response characteristic and a lifetime into consideration. Plastic has, however, a slight gas permeability.
The pressure equalizing hole applies atmospheric pressure as a reference pressure on the reference-pressure-operating-chamber-side side surface of the diaphragm. In addition, the pressure equalizing hole functions to prevent slight gas, which permeate from the fluid-pressure-operating-chamber-side side surface (the pressure-sensing-side surface) into the reference-pressure-operating-chamber-side side surface of the diaphragm due to the gas permeability of the diaphragm material, from affecting the reference pressure.
FIGS. 1A and 1B show a longitudinal sectional view of a conventional pressure equalizing hole 14 formed in a part of the inner surface of a connection terminal insertion hole 12 of an outer case 10, and a plane view thereof, respectively. As apparent from these figures, the pressure equalizing hole 14 is formed along all over a longitudinal side (having a maximum size L) of an outer peripheral surface in a cross section of an outer connection electrical terminal 16 inserted into an outer connection electrical terminal insertion hole 12.
In the above-structured conventional pressure switch, there is a possibility that, dust enters into the reference pressure operating chamber of the inner space of the outer case through the pressure equalizing hole and is adhered to the electrical contact means stored in the reference pressure operating chamber, and causes defectiveness of electrical conductivity in the electrical contact means, when the pressure switch is used in the airconditioner for an automobile.